1. Field of the Invention
The present invention relates to an optical signal processing system which converts a serial pulse train optical signal with transmission speed N to parallel pulse train electrical signals, and in particular to an optical signal processing system which converts a high-speed optical time-division multiplexing signal to electrical signals. The present invention is related in detail to an optical signal processing system which converts a high-speed optical signal to electrical signals by performing time-demultiplexing of the high-speed optical signal using a low-speed electrical signal.
2. Description of the Prior Art
Popularization of information communicating networks including the Internet has increased the amount of data to be transmitted, and in recent years, further increase of transmission capacity has been being required in optical communications. In order to increase the transmission capacity, both the time division multiplexing (hereafter abbreviated as TDM) system that multiplexes multi-channel optical signals to a time-serial optical signal and the wavelength division multiplexing (hereafter abbreviated as WDM) system that multiplexes channels through light of different wavelengths are employed. Specifically, an increase of transmission capacity is intended by decreasing the distance between wavelengths of channels transmitted through a transmission line using the WDM system and by increasing the transmission speed (bit rate) per time interval in each channel using the TDM system.
FIG. 1 is a configuration drawing of a conventional optical signal processing system that demultiplexes an optical time-division multiplexing signal (hereafter abbreviated as optical TDM signal) to electrical signals before multiplexing (e.g., refer to patent document 1: Gazette containing Japanese Laid-open Patent Application No. 8-181667 (paragraph numbers 0002 to 0004, FIG. 16)). In FIG. 1, optical fiber 100 transmits an optical TDM signal. Receiving part 10 consists of, for example, a high-speed PIN photodiode, whose input section is connected to optical fiber 100. The input section of synchronizing circuit 11 is connected to the output section of receiving part 10. The input section of electrical demultiplexing (hereafter abbreviated as DEMUX) circuit 12 is connected to the output section of synchronizing circuit 11.
The operation of such a system will be described below.
A WDM signal transmitted through a transmission line for optical communication is divided into each channel by an optical demultiplexer not shown in the drawing. An optical TDM signal for one channel of the WDM signal is transmitted through optical fiber 100 and is input to receiving part 10. For example, if it is assumed that the optical TDM signal is an optical TDM signal of 10 Gbps in which four optical signals of 2.5 Gbps each are subjected to time-division multiplexing, receiving part 10 receives the optical TDM signal of 10 Gbps, converts this signal to an electrical time-division multiplexing signal (hereafter abbreviated as electrical TDM signal), and then outputs it to synchronizing circuit 11.
Synchronizing circuit 11 extracts the clock signal from the electrical TDM signal and outputs the electrical TDM signal and clock signal to DEMUX circuit 12. DEMUX circuit 12 performs time-demultiplexing of the electrical TDM signal to return it to four electrical signals of a lower transmission speed of 2.5 Gbps and outputs them to a data processing part at a later stage not shown in the drawing.
FIG. 2 is a configuration drawing for another conventional example (e.g., refer to non-patent literature 1: “Ultra High-speed Optical Switching Technology” co-edited by Takeshi Kamiya and Yasuhiko Arakawa, the first edition, Baifukan Co., Ltd., July 1993). Here, the items equivalent to those shown in FIG. 1 are given the same signs and description for them will be omitted. In FIG. 2, serial-parallel converter 13 comprises optical switches, each having one-input and two-output terminals (hereafter abbreviated as 1×2 optical switches or simply optical switches), 13a, 13b, and 13c and the input section of serial-parallel converter 13 is connected to optical fiber 100. The input terminal of optical switch 13a is connected to optical fiber 100. The input terminal of optical switch 13b is connected to one of the two output terminals of optical switch 13a. The input terminal of optical switch 13c is connected to the alternative output terminal of optical switch 13a. 
The input terminals of receiving parts 14a to 14d are in turn connected to one of two output terminals of optical switch 13b, the alternative output terminal of optical switch 13b, one of two output terminals of optical stitch 13c, and the alternative output terminal of optical switch 13c respectively. The input section of clock extracting part 15 is connected to the output section of receiving part 14a and the output section of clock extracting part 15 is connected to both optical switches 13b and 13c. The input section of frequency multiplier 16 is connected to the output section of clock extracting part 15 and the output section of frequency multiplier 16 is connected to optical switch 13a. 
The operation of such a system is described below.
A 10 Gbps optical TDM signal, in which four optical signals of 2.5 Gbps each are time-division multiplexed, is input to optical switch 13a of serial-parallel converter 13 through optical fiber 100. Optical switch 13a performs time-demultiplexing of the optical signal by switching the connection to its output terminals at a speed of 5 GHz (½ the transmission speed of the optical TDM signal) and outputs optical TDM signals of 5 Gbps, this speed being lower than the speed of the input optical TDM signal, to optical switches 13b and 13c. 
Further, each of optical switches 13b and 13c performs time-demultiplexing by switching the connection to their output terminals at a speed of 2.5 GHz and outputs optical signals of 2.5 Gbps each to receiving parts 14a to 14d. That is, serial-parallel converter 13 performs time-demultiplexing of the 10 Gbps time-serial optical TDM signal, obtains four 2.5 Gbps parallel optical signals, and outputs them to receiving parts 14a to 14d respectively. Finally, each of receiving parts 14a to 14d converts an optical signal to an electrical signal.
Now, the operation of the switching control of optical switches 13a to 13c will be described. Clock extracting part 15 extracts a clock signal of 2.5 GHz, the phase of which is synchronized with the 2.5 Gbps electrical signal from receiving part 14a, from this electrical signal. Using this clock signal, connections of optical switches 13b and 13c are switched. In addition, frequency multiplier 16 (used as a frequency doubler in this case) changes the frequency of the clock signal to 5 GHz, twice the original frequency, and the connection of optical switch 13a is switched using this frequency.
Although the transmission speed of the optical TDM signal currently activated is 10 Gbps or so, increasing the transmission speed is desirable to expand the transmission capacity and research and development for speeds of 40 Gbps, 80 Gbps and 160 Gbps is underway.
However, the system shown in FIG. 1, has a problem in that it is difficult for receiving part 10 which converts an optical signal to electrical signals, synchronizing circuit 11 which handles electrical signals, and DEMUX circuit 12 to respond to speeds equal to or higher than 40 Gbps, and especially difficult to synchronize circuit 11 and DEMUX circuit 12 which handle electrical signals, to respond to such speeds.
On the other hand, for the system shown in FIG. 2, receiving parts 14a to 14d and clock extracting part 15 are enough to be capable of processing only such optical signals and electrical signals as thier transmission speeds are reduced to ¼ by serial-parallel converter 13. However, another problem exists wherein frequency multiplier 16 which controls the first stage optical switch 13a is required to process higher speed electrical signals thereby making it difficult for it to respond to speeds equal to or higher than 40 Gbps.